Apparatus for measuring the electrical conductivity of vascular fluids is of potentially great value of diagnosing and monitoring critical conditions of the heart, lungs and blood. Such apparatus typically comprises a catheter cardiac probe including a pair of ring-like electrodes longitudinally spaced apart along the catheter. A small voltage is applied between the two electrodes, and the resulting current flow provides a measure of the conductivity of the vascular fluid between the two electrodes. In typical operation, a bolus of hyptertonic saline solution having a conductivity greater than that of blood is injected upstream of the electrodes. The passage of the bolus through the heart is monitored by measuring downstream the perturbation in conductivity of the vascular fluids. The rate at which the bolus passes and the manner of its passage provide useful measures of cardiac output.
In contrast to relatively slow conventional techniques based on measurements of thermal dilution, measurements of conductivity dilution are sufficiently fast that the conductivity variation can be measured for each separate beat of the heart. In addition, measurements of conductivity dilution can be used to supplement measurements of thermal dilution to obtain valuable information concerning the thermal mass of the tissues surrounding the vascular system. A potentially valuable application of such apparatus involves a comparison of the conductivity perturbation with the thermal perturbation to provide a measure of potentially dangerous water build-up in the lungs.
Unfortunately, prior art techniques for measuring the conductivity of vascular fluids have been inaccurate and inconsistent. As a consequence, despite the fact that conductivity sensing techniques were first used to measure cardiac output nearly 80 years ago, this potentially valuable technique has long been neglected as inherently unreliable.